The clinical treatment of schizophrenia has long been defined by the dopamine hypothesis of schizophrenia, which holds that schizophrenia is a result of hyperactivity of dopaminergic neurotransmission, particularly in limbic brain structures such as nucleus accumbens (the mesolimbic dopamine system). Indeed, the positive symptoms of schizophrenia (hallucinations, delusions, thought disorder) are successfully treated with neuroleptics, which block dopamine receptors. However, such treatment is accompanied by the production of movement disorders or dyskinesias (extrapyramidal side effects), due to the blockade of nigrostriatal dopamine receptors. In addition, neuroleptics do not treat the negative symptoms of schizophrenia (social withdrawal, anhedonia, poverty of speech) which are related to a relative hypoactivity of neurotransmission in the mesocortical dopamine system and which respond to treatment by dopamine agonists.
Efforts to induce antipsychotic activity with dopamine autoreceptor agonists have been successful (Corsini et al., Adv. Biochem. Psychopharmacol. 16, 645-648, 1977; Tamminga et al., Psychiatry 398-402, 1986). Dopamine autoreceptor agonists produce a functional antagonism of dopaminergic neurotransmission by the reduction of neuronal firing and the inhibition of dopamine synthesis and release. Since dopamine autoreceptor agonists are partial agonists at postsynaptic dopamine receptors, they provide a residual level of stimulation sufficient to prevent the production of dyskinesias. Indeed, partial agonists are capable of functioning as either agonists or antagonists depending on the level of dopaminergic stimulation in a given tissue or brain region, and would therefore be expected to have efficacy versus both positive and negative symptoms of schizophrenia. Thus, novel dopamine partial agonists are of great interest for the treatment of schizophrenia and related disorders.